1. Field of the Invention
The present invention relates generally to fiber optic data transmission systems, and more particularly to an apparatus for detecting and determining the data rate of an incoming digital signal for use in a fiber optic work station capable of operation in a plurality of data rates without user or software intervention.
2. Description of the Prior Art
Fiber optics have been found to be particularly efficient for transmission of digital data between computer work stations. A present day work station configuration may employ multiple slave work stations, wherein advances in technology have permitted newly emplaced work stations to operate at a higher data rate then the earlier emplaced work stations. For example, while the older work stations may operate at a maximum of 307.2 Kbit/sec, a present day work station may operate at 1.8 Mbit/sec. It is desirable to utilize both the earlier and the technologically improved work stations on a common system. One prior art system has required the computer interface to interrogate each work station, read the operational speed in software, and downshift or upshift the data transmission rate accordingly. Thus, if there is a single work station operating at 307.2 Kbit/sec, the data transmission rate must be downgraded accordingly. It is desirable, however, that the improved work stations be indpendently capable of detecting the data rate of the incoming data so that it may be automatically adapted to the data transmission rate without software intervention. In particular, it is desirable that such rate recognition occur dynamically without significant loss of data.
An asynchronous circuit for dynamically recognizing rates of data transmission is taught by Bruce J. Miller in U.S. Pat. No. 4,584,719, Fiber Optic Work Station Data Link Interface. An asynchronous data transmission system requires that start and stop information be transmitted in addition to the data, since both the transmitter and receiver have to sense when valid data is being sent. This results in a reduction is efficiency, since the useful bit density is diminished and additional logic must be provided to decode the synchronizing data bits. The present invention operates synchronously and therefore does not require transmission of start and stop information. It is capable of recognizing the start and stop of data transmission directly from the encoded data. It has the advantage of keeping many diverse units in step with each other, since circuit performance is predictable at any given point and time, and also requires less logic in the individual device units.
The present invention is capable of detecting the incoming data rate and downshifting to match the clock rate on the system transmission lines without software or user intervention.